Specifically, learning Abilities if you have (Magic + Gift), (Divine + True Faith), (Farie + an invisible friend), or (Infernal + a convenient excuse): do they penalize each other, or do they penalize only other abilities learned in the same realm?
OK - so it looks like it's strictly on a per-realm basis: Magic abilities subtract from Magic abilities, Favored abilities don't penalize anything, and so on. That seems to match up with Rop:D (sort of), and RoP:I (better) -
However, in coming across Hedge Magic:
With that in mind: is there any written ruling that explicitly states that Abilities only penalize each other within a Realm? Currently I'm with a GM who is reading it that they penalize cross-realm.
OK, just read THIS thread, (https://forum.atlas-games.com/t/catnip-garlic/129/1) and figured out that there is an actual difference of preference between the authors in terms of whether or not Favored abilities affect the learning of other Supernatural Abilities. That's good to clear up. Still looking for any official wording on if they affect each other cross-realm.
I don't count as official word, but I can tell you, as the author of the relevant bits of the Realms books, that Rules As Intended are to treat each Realm as separate. Faerie Supernatural Abilities shouldn't count against Magic, Divine or Infernal, and so on. This is so that characters have multiple ways to develop over time-- magi who become interested in one of the other Realms can always explore it further if they choose to do so.
The rule about Favored Abilities scores penalizing neither favored nor other Supernatural Abilities was originally written without the bit about non-favored Abilities in Divine, and then I believe someone brought up the problem of that discouraging taking favored abilities as Virtues here on the forums, so I revised the wording for Infernal, expecting errata. I had thought I'd written Faerie to match Infernal.
If your storyguide prefers to keep the various Realm magics out of wizardly hands, that's certainly understandable. It gives greater range for companions, after all. And I can certainly see the logic in saying that a magus can't swear to follow a divine tradition and also follow an infernal tradition and a faerie tradition. But then again, if it makes sense in terms of the story, the rules should allow it.
Hey - yeah, I've been going back in time here on the forums for this one, and there are a few threads that touch on the question. Back in 2009 there was a bit of a discussion about it (https://forum.atlas-games.com/t/new-herbam-and-other-spells/157/1), where the conclusion seemed to be that they DO penalize each other (aside from Favored Abilities, which may or may not penalize anything but themselves, depending on the realm), but by 2014 your position seems to have solidified to the statement in the OP.